(1) Field of the invention
This invention relates to an oxygen-absorbing resin composition and a packaging container, a packaging material, a cap or a liner material having oxygen absorbability. More particularly, it relates to an oxygen-absorbing resin composition having a non-uniform multilayer distributed structure in which the stress produced by volume expansion through an oxidation reaction of an oxygen absorbing agent can be alleviated within the resin composition and also relates to a packaging material, a packaging container, a cap, and a liner material for caps which have an oxygen-absorbability, and excellent preserbability of the contents.
(2) Prior art
Metal cans, glass bottles and various plastic containers have heretofore been used as packaging containers. Among them, plastic containers have been used in a variety of applications owing to their light weights, shock resistance and cost.
However, though no oxygen is permitted to permeate through the walls of the metal cans and glass bottles, oxygen permeates through the walls of the plastic containers to an extent that cannot be neglected, arousing a problem from the standpoint of preserving the contents.
In order to prevent this, the walls of the plastic containers are formed in a multi-layer structure and in which one layer is composed of a resin having resistance against the permeation of oxygen, such as ethylene/vinyl alcohol copolymer.
An oxygen-absorbing agent has long been used in order to remove oxygen in the container. An example of using the oxygen-absorbing agent in the container wall has been disclosed in Japanese Patent Publication No. 1824/1987, according to which a layer obtained by blending an oxygen permeable resin with an oxygen-absorbing agent comprising chiefly a reducing substance is laminated on a layer that does not permit the permeation of the oxygen gas, in order to realize a multi-layer structure for packaging.
The oxygen-absorbing agent of the iron type features a large oxygen-absorbing rate and a large oxygen-absorbing capacity, and is advantageous from the standpoint of cost. When iron and the compounds thereof elute out into the content, however, the content loses flavor though the amount of elution may be very small.
In order to prevent the iron-type oxygen-absorbing agent from eluting into the content, as disclosed in Japanese Laid-Open Patent Publication No. 114371/1998, means has been employed in which a resin layer without blended with the oxygen-absorbing agent is applied onto the inner and outer surfaces of the resin layer blended with the iron-type oxygen-absorbing agent in order to prevent the iron-type oxygen-absorbing agent from being exposed.
It was, however, learned that the elution of iron or a compound thereof into the content is not sufficiently prevented by simply applying a thermoplastic resin layer without blended with an oxygen-absorbing agent onto both sides of the thermoplastic resin layer that is blended with the iron-type oxygen-absorbing agent.
That is, the iron-type oxygen-absorbing agent in the layer remains completely covered with the thermoplastic resin layers that are not blended with the oxygen-absorbing agent within not long periods of time after the production of the multi-layer plastic containers. When the multi-layer plastic containers are preserved for extended periods of time, however, particles of the iron-type oxygen-absorbing agent penetrate through the thermoplastic resin coating layers and are exposed to the outside of the coating layers. As the particles penetrate through, the oxygen-absorbing agent elutes into the content to deteriorate the flavor in addition to the factor of oxygen.
This tendency is especially conspicuous in the case of retorting sterilization.